In the past 30-35 years, the state of the art of polymer membrane-based gas separation processes has evolved rapidly. Membrane gas separation is of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications have achieved commercial success, including carbon dioxide removal from natural gas and from biogas and enhanced oil recovery, and also hydrogen removal from nitrogen, methane, and argon in ammonia purge gas streams. For example, UOP's Separex® membrane is currently an international market leader for carbon dioxide removal from natural gas.
The membranes most commonly used in commercial gas separation applications are polymeric and nonporous. Separation is based on a solution-diffusion mechanism. This mechanism involves molecular-scale interactions of the permeating gas with the membrane polymer. The mechanism assumes that in a membrane having two opposing surfaces, each component is sorbed by the membrane at one surface, transported by a gas concentration gradient, and desorbed at the opposing surface. According to this solution-diffusion model, the membrane performance in separating a given pair of gases (e.g., CO2/CH4, O2/N2, H2/CH4) is determined by two parameters: the permeability coefficient (abbreviated hereinafter as PA) and the selectivity (αA/B). The PA is the product of the gas flux and the membrane skin thickness, divided by the pressure difference across the membrane. The αA/B is the ratio of the permeability coefficients of the two gases (αA/B=PA/PB) where PA is the permeability of the more permeable gas and PB is the permeability of the less permeable gas. Gases can have high permeability coefficients because of a high solubility coefficient, a high diffusion coefficient, or because both coefficients are high. In general, the diffusion coefficient decreases while the solubility coefficient increases with an increase in the molecular size of the gas. In high performance polymer membranes, both high permeability and selectivity are desirable because higher permeability decreases the size of the membrane area required to treat a given volume of gas, thereby decreasing capital cost of membrane units, and because higher selectivity results in a higher purity product gas.
Polymers provide a range of properties including low cost, permeability, mechanical stability, and ease of processability that are important for gas separation. A polymer material with a high glass-transition temperature (Tg), high melting point, and high crystallinity is preferred. Glassy polymers (i.e., polymers at temperatures below their Tg) have stiffer polymer backbones and therefore let smaller molecules such as hydrogen and helium pass through more quickly, while larger molecules such as hydrocarbons pass through more slowly as compared to polymers with less stiff backbones. However, polymers which are more permeable are generally less selective than are less permeable polymers. A general trade-off has always existed between permeability and selectivity (the so-called polymer upper bound limit). See Robeson, J. Membr. Sci., 62 (2): 165 (1991); Robeson, et al., Polymer, 35 (23): 4970 (1994). Over the past 30 years, substantial research effort has been directed to overcoming the limits imposed by this upper bound. Various polymers and techniques have been used, but without much success.
Cellulose acetate (CA) glassy polymer membranes are used extensively in gas separation. Currently, such CA membranes are used for natural gas upgrading, including removal of carbon dioxide. Although CA membranes have many advantages, they are limited in a number of properties including selectivity, permeability, and chemical, thermal, and mechanical stability. One of the immediate challenges that need to be addressed in CA polymer membranes is achieving higher selectivity with equal or greater permeability.
In order to enhance membrane selectivity and permeability, a new type of mixed matrix membranes (MMMs) has recently been developed. Almost all of the MMMs reported to date in the literature are hybrid blend membranes comprising insoluble solid domains such as molecular sieves or carbon molecular sieves embedded in a polymer matrix. Some examples can be seen in the following recent patents and published patent applications: U.S. Pat. No. 6,626,980 B2; US 2003/0220188 A1; US 2005/0043167 A1; U.S. Pat. No. 6,562,110 B1; U.S. Pat. No. 6,755,900 B2; U.S. Pat. No. 6,500,233 B1; U.S. Pat. No. 6,503,295 B1; and U.S. Pat. No. 6,508,860 B1. They combine the low cost and easy processability of the polymer phase with the superior gas separation properties of the molecular sieve phase. These membranes have the potential to achieve higher selectivity with equal or greater permeability compared to existing polymer membranes, while maintaining their advantages. In contrast to the many studies on conventional polymers for membranes, only a few attempts to increase gas separation membrane performance with mixed matrix membranes of zeolite and rubbery or glassy polymers have been reported.